1. Field of the Invention
This invention relates to a magnetoresistive head, for a magnetic recording apparatus and particularly a magnetic disk apparatus, utilizing the magnetoresistive effect.
2. Description of the Related Art
A magnetoresistive head is currently known as a magnet head which can read data from the magnetic surface of a magnetic recording medium with high sensitivity. A magnetoresistive head converts a magnetic signal on a recording medium into an electrical signal by utilizing the phenomenon whereby the electrical resistance of a magnetoresistive film varies according to the direction of magnetization.
A primary problem with conventional magnetoresistive heads is that Barkhausen noise would occur as a domain wall existing in the magnetoresistive film is irregularly moved due to a signal field.
U.S. Pat. No. 4,103,315 discloses, as an attempt to eliminate a domain wall of a magnetoresistive film and so suppress noise, a concept of utilizing the antiferromagnetic-ferromagnetic exchange coupling by causing an unidirectional magnetic bias field to occur due to an antiferromagnetic layer mounted entirely on one surface of the magnetoresistive film.
Here in the present specification, a layer for applying a longitudinal bias field to a magnetoresistive film in order to suppress any domain wall in the magnetoresistive film is defined as "a domain suppressing layer". The term "exchange coupling" means to make the spin ordering of a ferromagnetic film coincide with the spin ordering of an antiferromagnetic film at the interface region of these films.
In the structure disclosed in U.S. Pat. No. 4,103,315, since the antimagnetic film is laminated entirely over one surface of the magnetoresistive film to provide a domain suppressing layer, the exchange coupling would become increased. As a result, a relatively intense signal field from the magnetic recording medium is needed to turn the direction of magnetization of the magnetoresistive film, thus lowering the sensitivity of the magnetoresistive film with respect to the signal field.
The foregoing problem in the prior art was solved by a concept of patterning the domain suppressing layer and locating the patterned domain suppressing layer only over each of opposite ends of the magnetoresistive film. This concept is disclosed in U.S. Pat. No. 4,663,685. An antiferromagnetic film, serving as the domain suppressing layer for applying a bias field in the longitudinal direction of to the magnetoresistive film, is located only over each of opposite ends of the magnetoresistive film to keep the film end in a single domain state, thus causing a single domain state also in a magnetosensitive portion disposed at the center of the magnetoresistive film.
This prior art has the following problems. The antiferromagnetic film acting as a domain suppressing layer must be formed over each end of opposite ends of the magnetoresistive film which is previously patterned in a predetermined size. On the other hand, an exchange coupling must be caused between the antiferromagnetic film and a ferromagnetic film, which serves as the magnetoresistive film. Since this exchange coupling interaction runs only a short distance (about a single atom layer), the two films must be formed in atomically intimate contact with each other. Therefore, the magnetoresistive film surface contaminated such as by oxides must be cleaned before having the antiferromagnetic film formed over it.
Partly because the cleaning step must be added and partly because a magnetoresistive film usually has a thickness of hundreds of angstroms, which is very small, the magnetoresistive film is highly prone to damage during this cleaning step. Therefore, the magnetic characteristic is impaired. If such cleaning is not uniform, the magnetoresistive film will vary in thickness so that there is a difference in magnetic characteristic between various magnetic heads. If the cleaning is insufficient, the strength of exchange coupling between the magnetoresistive film and the antiferromagnetic film will be inadequate so that there is a possibility that each of opposite ends of the magnetoresistive film may not become a single domain. When it does not become a single domain, there will exist a domain wall in the magnetoresistive film so that Barkhausen noise occurs due to the irregular movement of the domain wall. The resulting magnetic head is not suitable for use in a high-density magnetic recording device.
Another problem with the prior art is that since the antiferromagnetic film is formed directly on the magnetoresistive film, the material for the antiferromagnetic film is limited to such a material which has a lattice structure similar to that of the magnetoresistive film. The exchange coupling between a magnetoresistive film, which is a ferromagnetic film, and an antiferromagnetic film is nonstable; it is therefore necessary to use the same lattice structure for both the ferromagnetic film and the antiferromagnetic film so that a fine exchange coupling is formed.
Further, since the antiferromagnetic film serving as a domain suppressing layer is formed directly over the magnetoresistive film, the strength of exchange coupling between the magnetoresistive film and the antiferromagnetic film is necessarily determined only according to the material of both the magnetoresistive film and the antiferromagnetic film. On the other hand, the strength of exchange coupling depends on the longitudinal bias field of the magnetoresistive film and is a quantity which needs to be controlled to an optimum value for the structure of a magnetoresistive head. With this prior art, however, only by changing the material, the strength of exchange coupling can be controlled.